Forestry's contribution to Hg bioaccumulation in freshwaters: assessment of the available evidence


Mercury (Hg) levels are alarmingly high in fish from lakes across Fenno-Scandia and northern North America, with distinct regional hotspots. Long-range transport by air pollution is the ultimate origin of most of the Hg in fish. Most of the Hg that arrives in the rain remains in the soil. This soil retention has protected aquatic ecosystems from the full effect of the Hg in atmospheric deposition. The resulting Hg build up in the soils may be a threat to the soil biota. But it is definitely a threat to fish, since anything that increases rates of transfer from soils to water will make the problem even worse in fish and other aquatic biota. Of the different forms of Hg, methylmercury (MeHg) is by far the most susceptible to biomagnification in the aquatic food chain, and the factors that promote methylation (such as more reducing conditions due to elevated water tables) are of particular concern. Recent studies have shown that forest harvest operations are associated with increased levels of total Hg (Hgtot) and MeHg in run-off and the biota. While only a few relevant studies have been conducted, they are consistent in pointing to forestry operations increasing the total loading of mercury to aquatic ecosystems. In a set of Canadian studies on uptake by the biota, increases in fish and periphyton concentrations between two- and fourfold were seen. When it comes to Hg in run-off, the smallest effect of forest harvest on Hg outputs was a doubling of Hgtot and MeHg outputs. Several other boreal studies found concentration increases in Hgtot by a factor of two to ten. Increases in MeHg concentrations were generally larger, between four- and twentyfold in a given year. In one case, such increases were associated not with harvest, but only with creation of a temporary skidding road. The duration of increased MeHg concentrations appears to persist for at least five years. From these studies we estimate that 10-25 percent of the mercury now in the fish of forested, high-latitude landscapes can be attributed to forest harvest. The assumptions underlying this estimate are that forest harvest increases the uptake of Hg in fish by a factor of two to four for a period of ten years, and that one percent of the landscape is harvested each year. If thinning operations also have a similar affect on bioaccumulation, and there is one thinning before final harvest, the figure for forestry's 'share' rises to 15-35 percent. This result should be evaluated in the light of two important considerations. One is that forest operations raise Hg/MeHg outputs to levels that are similar to those seen from wet-lands. The second is that forest harvest is not what created the elevated Hg levels in the soils. And even if harvest increases the output from soils, we do not know the degree to which other forestry practices might be increasing Hg retention while the forest is growing - for instance due to increased mor layer thickness, or lowered water tables. When it comes to what can be done to mitigate forest harvest impacts, most of the current recommendations in Sweden appear helpful based on current knowledge, such as leaving riparian buffer zones, reducing stream disturbance with bridges, and avoiding operations in moist areas. Two types of recommendation warrant further consideration. One is the creation of humus traps, since they might promote methylation. Restoration of wetlands and reduction of the degree of drainage will also increase Hg/ MeHg loadings to aquatic ecosystems, but the magnitude of the effect will depend on the type of wetland and the local geochemistry. However accurately these conclusions reflect the current state of knowledge, it must be emphasized that there are very few studies specifically focused on this issue. Therefore there is an urgent need to advance our understanding of how forestry influences the cycling of Hg, its bioaccumulation in aquatic ecosystems, and Hg affects on the soil biota.

Medarbetare: Sofie Hellsten, John Munthe

Nyckelord: Forestry's contribution, Hg bioaccumulation, freshwaters

Typ: Artikel

År: 2009

Rapportnummer: A1855

Författare: Kevin Bishop, Craig Allan, Lage Bringmark, Edenise Garcia, Sofie Hellsten, Lars Högbom, Kjell Johansson, Anja Lomander, Markus Meili, John Munthe, Mats Nilsson, Petri Porvari, Ulf Skyllberg, Rasmus Sorensen, Therese Zetterberg, Staffan Åkerblom

Publicerad i: Kungl. skogs- och lantbruksakademiens tidskrift, 2009. Nr 1, årgång 148, pp. 9-23